Build junit_runner in the exec configuration

junit_runner is a test-support tool, like process_wrapper and the
coverage collector, so build it the way they are -- in the exec
configuration. As a target-configuration dependency it inherited
target-only settings it can't satisfy: a workspace that pairs a custom
#[global_allocator] with cc_common.link (rules_rust's own
test/integration/cc_common_link_with_global_alloc) failed to link the
runner, because it has no global allocator of its own to supply the
allocator symbols.

Building it for the exec platform keeps it off the target configuration,
which is where it belongs anyway -- it's the test's launcher, it runs
wherever the test is executed, and it has no reason to match the tested
crate's build settings.

Author: jlnhws

rust/private/rust.bzl

@@ -1043,7 +1043,12 @@ _RUST_TEST_ATTRS = {
     "_junit_runner": attr.label(
         default = Label("//util/junit_runner"),
         executable = True,
-        cfg = "target",
+        # Built for the exec platform, like the other test-support tools
+        # (process_wrapper, collect_coverage). This keeps the runner off the
+        # target configuration, so it doesn't inherit target-only settings such
+        # as a custom #[global_allocator] or cc_common.link, which it has no way
+        # to satisfy and which would otherwise fail to link.
+        cfg = "exec",
     ),
 } | _COVERAGE_ATTRS | _EXPERIMENTAL_USE_CC_COMMON_LINK_ATTRS